Apparatus and method for reducing image artefacts

ABSTRACT

The invention relates to an X-ray apparatus for forming X-ray images, which apparatus includes an X-ray detector ( 4 ) for the conversion of X-rays into electrical signals, a detector exposure unit ( 5 ) for the emission of electromagnetic radiation in dependence on how first and second exposure parameters, the value of the first exposure parameters being defined by the acquisition mode whereas the second exposure parameters are not defined by the acquisition mode, and also a control unit ( 13 ) for changing and controlling at least one second exposure parameter of the detector exposure unit upon a change of the acquisition mode.

The invention relates to an apparatus and a method for reducing imageartifacts for X-ray detectors.

X-ray detectors are devices, which convert X-rays into a typicallyelectronic signal, which can be evaluated. An X-ray signal acquired atan instant is referred to as an X-ray image. An X-ray image consists ofat least one X-ray signal value. A temporal succession of X-ray imagesis referred to as an X-ray image sequence.

X-ray detectors are used inter alia in imaging X-ray apparatus in themedical field. Contemporary detectors utilize, for example, a conversionlayer which consists mainly of a scintillation material which absorbsthe X-rays and emits optical light quanta essentially in proportion tothe incident overall energy of the X-rays. The scintillation material istypically attached, by way of an optically transparent adhesive, to aphotodiode, which is based on a semiconductor material such as silicon,or to a one-dimensionally or two-dimensionally structured photodiodearrangement, or is vapor-deposited thereon. The photodiode arrangementmay additionally include electronic devices for the reading out,amplification and digitization of the photodiode signals. Thephotodiodes absorb optical quanta and convert such quanta into chargecarriers. The charge carriers are read out and can be converted into avoltage. An X-ray signal value is also referred to as a pixel value. Apixel typically represents an image point.

Other X-ray detectors may be made, for example, of directly-convertingmaterials. An X-ray detector may be capable of counting individual X-rayquanta or of integrating the incident X-ray energy.

The photodiode or the photodiode arrangement of the X-ray detector maybe made of silicon, in which case crystalline, polycrystalline oramorphous silicon may be used. X-ray detectors having a particularlylarge surface area, for example, as used in contemporary radiography,can be manufactured when use is made of amorphous silicon (a-Si:H). Whenthe light quanta emitted, for example, by the scintillation material areconverted, charge carriers are raised from the valence band of thea-Si:H to the conduction band. The free charge carriers can be read out.The a-Si:H, however, has a large number of local capture states betweenthe bands, in which capture states charge carriers are retained for sometime before they are emitted again, for example, due to thermalexcitation. In the case of X-ray image sequences the foregoing causesthe X-ray detector to exhibit so-called afterglow, which is dependent onthe local radiation history. As a result, each pixel contains signalcomponents, which do not belong to the charge carriers directlygenerated in the instantaneous irradiation period. If a part of thedetector was strongly irradiated in a preceding image and is exposed toa small number only of X-ray quanta in the instantaneous irradiationperiod, the afterglow signal may have a decisive effect on the overallsignal. This gives rise to partly unusable images. Such signalcomponents of the X-ray image are also referred to as image artifacts.Such incorrect images cannot be tolerated, that is, notably in the caseof irradiation of human patients, because the applied X-ray dose shouldbe converted as well as possible into usable medical data and may not beapplied unnecessarily.

An X-ray detector is often used to acquire temporal sequences of X-rayimages. The values of the acquisition parameters, such as imageacquisition time, detector resolution and X-ray exposure time, governthe acquisition mode in which the sequence is acquired. It is oftendesirable to change at least one acquisition parameter in the course ofan X-ray image sequence so as to achieve adaptation to the changingrequirements, for example, during an interventional examination, thatis, for example, in the case of the X-ray images acquired during asurgical intervention in a patient. The changing of one acquisitionparameter leads to another detection mode.

WO 98/01992 discloses an X-ray apparatus, which includes not only theX-ray detector and an X-ray source, but also an additional source ofradiation, which emits electromagnetic radiation. Such an additionalradiation source (also referred to as a bias light source) can be usedto fill, prior to and/or during the X-ray exposure, the capture statesin the a-Si:H by additional application of a homogeneous light signal,so that the signal generated by the X-ray exposure ultimately cannotlose charge carriers to the already filled capture states, meaning thatthe X-rays are also directly converted into readable charge carriers.The additional signal emitted by the filled capture states ishomogeneous to a high degree and does not give rise to a noticeabledegradation of the image. WO 98/01992 explains that a change of theacquisition parameters (for example, image acquisition time, detectorresolution etc.), that is, a change of the acquisition mode, may lead toa change of the dynamic equilibrium of the charge carriers moved to thecapture states by the additional light and the charge carriers emittedthereby. This necessarily leads to image artifacts, because thebackground signal from the capture states forms part of the offsetsignal to be subtracted. The absolute value of the offset signal,however, is determined in the state of the equilibrium. In order toavoid image artifacts, WO 98/01992 proposes to abstain from changing theacquisition parameters; this means a distinct limitation of theusability and flexibility of the apparatus.

In given circumstances, for example, in the case of an intervention, itmay be necessary to change from one acquisition mode to another. Thisleads to an imbalance of the ratio of the captured and the emittedcharge carriers and hence to image artifacts after such a change.

Therefore, it is an object of the present invention to provide anapparatus and a method whereby undesirable image artifacts can bereduced for an X-ray detector.

The object is achieved by means of an X-ray apparatus for theacquisition of X-ray images, which apparatus includes an X-ray detectorfor the conversion of X-rays into electrical signals, a detectorexposure unit for the emission of electromagnetic radiation independence on first and second exposure parameters, the value of thefirst exposure parameters being defined by the acquisition mode whereasthe second exposure parameters are not defined by the acquisition mode,and also a control unit for changing and controlling at least one secondexposure parameter of the detector exposure unit upon a change of theacquisition mode.

In order to obtain an as good as possible image, in the case of digitalX-ray detectors two essential corrections are performed after theacquisition: first an offset signal is subtracted and subsequently gainstandardization is performed. The offset signal is composed of interalia leakage currents and charge carriers emitted from the capturestates. Furthermore, there are also afterglow effects in thescintillation material. The offset image, that is, the offset signalsfor each pixel in the relevant acquisition mode, are measured in thesteady state and stored so that during the acquisition sequence it canbe used to subtract the offset signal from the image signal. Theacquisition mode is determined by the acquisition parameters.Acquisition parameters are, for example, the image acquisition time, thedetector resolution and the X-ray exposure time. The X-rays are appliedto the X-ray detector typically in a pulsed fashion. In that case thereis time between the pulses to read out the pixel signals and also for asubsequent electronic reset during which the residual charge is drainedfrom the photodiode. During the reset operation preferably theadditional detector exposure unit which emits bias light is activated,so that the charge carriers produced by the detector exposure, not beingexcited in a capture state, are also drained. It is possible to utilizebias light of a wavelength such that the charge carriers cannot beraised to the conduction band. Such a bias light can be appliedcontinuously.

When the state density function of the capture states is in a non-steadystate, the offset signal measured in the steady state does not representthe instantaneous offset value. When the new steady state is reached,after the offset subtraction there will be a signal background whichleads to additional noise and a reduction of the dynamic range. In manycases the image will no longer be suitable for medical evaluation.

The gain normalization ensures that a uniformly irradiated X-raydetector produces a uniform output signal, irrespective of theproperties of the detector pixels. The quality of the gain normalizationmay also be adversely affected by a non-steady state of the capturestates.

A change of an acquisition parameter typically gives rise to anon-steady state in the ratio of the charge carriers excited by theadditional illumination from the valence band in a capture state to thecharge carriers emitted again from the capture states. When theacquisition sequence with the new acquisition parameters lasts longenough, a new steady state will be asymptotically reached.

The value of first exposure parameters of the detector exposure unit isdefined by the acquisition mode. For example, the bias light pulsespacing must be changed in conformity with the image acquisition timewhen the bias light pulse is to occur in the reset phase. The values ofsecond exposure parameters, such as the exposure intensity or theexposure time, are not defined by the change of the acquisition mode. Afurther second exposure parameter is the exposure wavelengthcomposition. The wavelength composition of the bias light can becontrolled when at least two bias light sources of different wavelengthor a means for changing the wavelength of a light source are available.

The problem posed by the occurrence of image artifacts after a change ofthe acquisition mode is dealt with in accordance with the invention inthat at least one second exposure parameter of the detector exposureunit is changed and controlled upon a change of the acquisition modewhich is determined by exposure parameters, that is, in such a mannerthat the new steady state to be reached deviates as little as possiblefrom the previous steady state, thus producing a small artifact signalonly.

In order to carry out the changing and the control of the exposureparameters, use is made of a control unit which has knowledge of theexposure parameters before and after the change of mode, so that it canperform the appropriate control in respect of the changing andcontrolling of the exposure parameters of the exposure unit.

In the case of nuclear medical examinations, a radioactive markingmaterial is injected into the patient so that the patient himselfbecomes an X-ray source. Radiographic X-ray apparatus, however, have anX-ray source for X-ray exposure of the patient. This is described inclaim 2.

The control unit is preferably also suitable to control the X-raydetector and an X-ray source which possibly forms part of the X-rayapparatus. This offers the advantage that the control of all systemcomponents is incorporated in one component and that correspondingparameters are available for all control and regulating operations. Theadditional control of components of the X-ray apparatus is disclosed inclaim 3.

The acquisition parameters governing an acquisition mode include theimage acquisition time, the X-ray exposure time and the detectorresolution. This is dealt with in claim 4.

The exposure intensity, the exposure time and the exposure wavelengthcomposition are particularly expressive exposure parameters. This isdisclosed in claim 5.

In accordance with the invention it is advantageous when the changingand control of a second exposure parameter, for example, the exposuretime, take place in proportion to the changing of an acquisitionparameter, for example, the image acquisition time. This is becausewhen, for example, the image acquisition time is prolonged, the integralbias light intensity is reduced in proportion to the prolongation. Acorresponding proportional prolongation of the exposure time results inthe same integral bias light intensity as before the changing of theacquisition parameter and hence in a substantially the same steady stateof the capture state density function in the new exposure mode. Thisproportional change is dealt with by claim 6.

Furthermore, in accordance with the invention it is advantageous whenthe changing and control of the exposure parameters after a change ofthe acquisition mode take place as a function of time. As has alreadybeen described, there are additional effects due to the scintillationmaterial or other detector elements. In order to counteract such effectsor to achieve a particularly advantageous approach for reaching a steadystate, the exposure parameters are controlled as a function of time,that is, from one image to the next the second exposure parameters areadjusted differently after the changing of at least one acquisitionparameter. The corresponding variations in time of the exposure can bedetermined experimentally, for example, by means of defined calibrationimages formed on the detector. The control should converge towards aconstant final value. This is disclosed in claim 7.

It is particularly advantageous when the changing and control values ofthe exposure parameters which have been determined in advance are storedin a table which can be read by the control unit, so that the changingand control for a given change of the acquisition mode take place bymeans of the values from this table. This is described in claim 8.

As has already been stated, an X-ray detector typically operates in apulsed mode and the detector exposure unit is active only in the resetphases, thus ensuring the draining of the charge carriers produced. Thisadvantageous embodiment is disclosed in claim 9.

Claim 10 discloses a method in accordance with the invention forenhancing the image quality. The X-ray detector is irradiated by meansof X-rays in a pulsed or continuous manner. Additionally, the X-raydetector is exposed to bias light from the detector exposure unit. Thisexposure can take place during the continuous irradiation by means ofX-rays within the phases in which the X-ray detector is not exposed toX-rays, that is, preferably in the reset phase. In the case of a changeof the acquisition mode, a control unit changes and controls at leastone second exposure parameter in such a manner that the steady state ofthe capture states can be adjusted with as little change as possible.The charge carriers generated in the detector pixels are read out andconverted, for example, into a current signal or a voltage signal. Thesignals thus acquired can also be digitized.

An embodiment of the invention will be described in detail hereinafterwith reference to some Figures. Therein:

FIG. 1 is a diagrammatic representation of an X-ray apparatus inaccordance with the invention,

FIG. 2 is a diagrammatic representation of a part of a two-dimensionalphotodiode arrangement,

FIG. 3 is a diagrammatic cross-sectional view of a detector of the X-rayapparatus in accordance with the invention, which detector is providedwith a bias light source,

FIG. 4 shows a part of a time diagram for controlling the X-rayapparatus in accordance with the invention,

FIG. 5 shows a variation of the mean value, corrected by means of anoffset signal, of a dark image before and after the changing of anacquisition parameter, and

FIG. 6 shows the variation of the mean value of FIG. 5 after thechanging and control of the exposure parameter in accordance with theinvention.

FIG. 1 shows an X-ray apparatus, which includes an X-ray source 1, whichirradiates an object 2, notably a patient to be radiologically examined,by means of an X-ray beam 3. X-rays which are not absorbed are convertedinto an electronic image signal IS by an X-ray detector 4. The imagesignal is applied to an image processing unit 10, which outputs an imagesignal current PS which corresponds to a processed X-ray image. Theprocessed X-ray image can be stored in a storage medium 12 and/or bedisplayed on a display screen 11. A bias light source 5 is arrangedbehind the detector 4 and exposes the detector to bias light inconformity with the exposure parameters imposed by the control unit 13.The control unit 13 in this embodiment controls not only the bias lightsource 5 but also the X-ray detector 4 and the X-ray source 1, that is,the latter via the high-voltage generator 14. The control unit 13 alsoincludes a memory 15 which is used to store data and tables for thechanging and control of parameters of the X-ray apparatus (for example,X-ray apparatus acquisition parameters and exposure parameters) and toread out such data and tables at a later stage again.

The bias light source 5 may be formed in known manner, for example, as amatrix of LEDs. An additional diffuser (not shown) ensures that theexposure is very homogeneous. A further embodiment consists of a matrix,which comprises two types of LEDs which are arranged, for example, in acheckerboard fashion and which emit light of a different respectivewavelength. This enables particularly flexible variations of thedetector exposure and hence constitutes an advantageous possibility forcounteracting image artifacts.

FIG. 2 is a diagrammatic representation of a part of a two-dimensionaldetector arrangement. The Figure shows 3×3 photodiodes 21. X-raydetectors may have a matrix size of 100×200, 1024×1024, 2000×2000photodiodes or another size. The photodiodes 21 absorb incident opticallight quanta and convert these quanta into charge carriers. The rowdriver circuit 25 controls the switching elements 23 (in this caseformed by switching transistors) via address leads 24 and the collectedcharge carriers of the photodiodes of one row are applied, when theswitching elements 23 are switched on, to integrating amplifier elements26 via respective read-out leads 22; finally, a multiplexer circuit 27successively switches through the individually present voltages.Possibly after further steps, for example, digitization, the imagesignal current IS is obtained. The row driver circuit is coupled to thecontrol unit 13, thus enabling control of the row sequencing.

FIG. 3 is a detailed cross-sectional view of the part A (denoted bydashed lines in FIG. 1) of the X-ray detector 4 and the bias lightsource 5 arranged behind the detector. The scintillator 7 absorbsincident X-rays and emits light quanta. The Figure shows how lightquanta emitted by the bias light source 5 can be incident, directly orindirectly, on the photodiode structure 21 via a preferably transparentsubstrate 30, for example, a substrate made of glass.

FIG. 4 shows a part of a time diagram of an X-ray apparatus. The line Aof the diagram shows the image triggering; the ascending arrow indicatesthat in this case ascending edge triggering is concerned. The imageacquisition time, that is, the period of time elapsing between two imagetrigger pulses, is denoted by the reference T_(B). The line B of thediagram shows the duration of the exposure by the detector exposureunit, which duration is denoted by the value T_(L). The line C of thediagram indicates the duration of the electronic reset (denoted by thereference T_(R)) during which all charge carriers are drained from thephotodiodes. The line D of the diagram shows the X-ray exposure time(referred to as T_(X)); the line E of the diagram shows the image signalread-out phase during which the X-ray image signals are read out duringthe read-out period T_(A), and the line F of the diagram shows the imagetransmission time during which the image signals are applied to theimage processing unit (transmission time T_(T)). Granted, the distancein time between the bias light pulses is also an exposure parameter.However, because the bias light pulse in the preferred embodiment canoccur only during the electronic reset phase, the distance in time ofthe bias light pulses is an exposure parameter whose value is determinedby the acquisition mode. When the image acquisition time T_(B) changes,the value of the distance in time between the bias light pulses mustalso change in order to ensure that the bias light pulses still occurwithin the reset phase. In the present example the bias light pulsespacing is defined as the image acquisition time T_(B). Such exposureparameters, imposed by the acquisition mode, form part of the group offirst parameters which are not available for the control and regulatingby the control unit. TABLE 1 Various acquisition modes of a digitalX-ray detector used for cardiac examinations. Effective Max. imageDetector image rate Max. Tx No. Mode resolution size [mm²] [images/s][ms] 1 Full image, 1 × 1 176 × 176 30 19.5 pulsed 2 Full image, 2 × 2176 × 176 60  8.0 combined, pulsed 3 Full image, 1 × 1 176 × 176 30continuous continuous 4 Zoom, 1 × 1 110 × 110 60  6.5

Table 1 shows various acquisition modes for a digital X-ray detector,which has been configured especially for cardiac examinations. Thisdetector has an active surface area of 176×176 mm². Column 1 containsthe number of the acquisition mode of the present selection. Column 2provides a brief description of the acquisition mode. In the case of“full image”, all pixels contribute to the image; in the case of “zoom”,only a limited part of the photodiode arrangement contributes to theimage. “Pulsed” means that the X-rays are emitted in a pulsed fashion bythe X-ray source 1 whereas “continuous” means that X-rays are emittedcontinuously. “Combined” describes an acquisition mode in which aplurality of pixels are combined so as to form one image point; in thiscase 2×2 pixels are combined so as to form one image point. In thecombined mode reading out can be faster and the image points have abetter signal-to-noise ratio. Column 3 indicates the detectorresolution, that is, it specifies how combination takes place. Column 4shows the effective image size in mm². Column 5 states the maximum imagerate, that is, the maximum number of X-ray images that can be read outper second in the case of a sequence acquisition. The last columnindicates the maximum X-ray exposure time T_(X) in ms at the maximumimage rate.

Different acquisition parameters are adjusted in the various modes. Forexample, it is possible to combine photodiodes so as to provide aneffectively larger image point at the photodiode level. This offersspecial advantages for applications involving a smaller applied X-rayrate per image; the larger image point then means a less favorableresolution, but also a better signal-to-noise ratio. In the mode 2 withcombined pixels a higher maximum image rate per second is obtainedbecause of the smaller number of image signal values to be read out. Onthe basis of the read-out time T_(A) and the reset time T_(R) a maximumX-ray exposure time T_(X) per image is obtained for a given image signalrate. Table 1 is to be understood as an example of a set of acquisitionmodes wherebetween switching over can take place. The Table does notinclude acquisition parameters such as the X-ray pulse intensity, theanode voltage of the X-ray tube, etc.

It is notably when the image acquisition time T_(B) is changed thatimage artifacts occur due to the fact that in the new mode the ratio ofthe charge carriers excited by the bias light in capture states to thecharge carriers emitted thereby has not yet been stabilized. The imageacquisition time T_(B) then increases, the distance between the biaslight pulses is also increased and on average fewer charge carriers incapture states are excited. Because the emission of captured chargecarriers satisfies, like all decay processes, an exponential law(intensity=A.exp(−t/τ, where A is a proportionality factor and t is thetime elapsed since the beginning of the process), and because the timeconstant τ then occurring is large relative to the image acquisitiontime T_(B), at the beginning of the new mode even more charge carriersare emitted than in the stabilized state. An offset-corrected dark imagethen exhibits an additional artifact signal after the change of mode,which artifact signal asymptotically approaches zero in the ideal case.The changing and control of at least one second exposure parameter ofthe detector exposure unit (for example, the exposure time T_(L)) inaccordance with the invention, so that the integral light intensityapplied to the detector by means of the exposure unit does not change,reduces the artifact signal.

FIG. 5 shows the behavior of examples of artifact signals withoutadaptation of a second exposure parameter in accordance with theinvention. The value of the offset-corrected dark image signal which hasbeen averaged across the detector is indicated therein in digitaldetector value units (LSB) relative to the image number (FN). It isshown for 20 images in two sequences that the change of mode took placefrom the image 9 to the image 10. The curve K1 shows the value for achange of the image acquisition time T_(B) from 135 ms to 565 ms and thecurve K2 belongs to a change of mode in which the pixel size waschanged, that is, a combination of pixels so as to form image points. Inthe case of a combination of 2×2 pixels the image acquisition time T_(B)is then reduced to approximately one half, because the column pixels canbe simultaneously read out. The bias light pulse duration T_(L), being asecond exposure parameter, was not changed by the change of mode.

FIG. 6 shows curves K3 and K4 which have been measured in the samecircumstances as the curves K1 and K2; the curves K1 and K3 belong tothe same change of mode like K2 and K4. The bias light pulse time T_(L)was changed in proportion to the change of the image acquisition timeT_(B) in accordance with the invention; thus, upon the change from 2×2combined pixels to non-combined pixels a doubling of the bias lightpulse duration T_(L) was selected. It appears that in the case of thechange of the image acquisition time T_(B) a reduction of the artifactsignals to very small residual artifacts could be achieved and that inthe case of the change of the pixel size a less pronounced reduction ofthe artifact signal was obtained. This is due to the fact that othereffects additionally contribute to the artifact signal in the case of achange of the pixel size.

A proportional prolongation of the bias light pulse duration T_(L) withthe same integral light quantity as in the preceding mode typicallyleads to a steady state which is not exactly the same, because othercapture state density functions are also obtained due to the changedtime intervals between the pulses, since a larger share of capturestates has been vacated in the longer time interval. Consequently, thenext bias light pulse encounters a different constellation of freecapture states. Therefore, the dynamic steady state is not exactly thesame for the same integral bias light exposure. In order to improve thechanging, a change can be performed which is not directly proportionalto the change of the image acquisition time T_(B) or the change can becontrolled in such a manner that the new steady state is reached in anoptimum fashion. When a longer or shorter bias light pulse durationT_(L) is adjusted for a brief period of time, which duration slowlyconverges towards the final value, or when the bias light pulse durationT_(L) is adjusted so as to fluctuate at increasingly smaller distancesfrom and around the final value, the offset-corrected signal variationcan be controlled more accurately to the zero line. Change values thuscontrolled result in even better artifact suppression. The exact valuesof a change controlled in this manner can be determined in the course ofthe detector calibration. The detector calibration is a necessary stepto determine the parameters of the detectors from defined measurementswith and without X-ray exposure, which parameters are required at alater stage for the further processing of the image signals, that is,the offset value for each pixel, the gain value for each pixel, etc.

Overall, an improved X-ray apparatus is obtained which includes an X-raydetector with a detector exposure unit, the detector exposure unit beingcontrolled by a control unit in such a manner that upon a change ofacquisition mode there is achieved a reduction of image artifacts incomparison with the present state of the art. The image qualityincreases and enables a change of mode without risking the formation ofX-ray images which are no longer suitable for medical evaluation.

1. An X-ray apparatus for forming X-ray images, which apparatus includesan X-ray detector (4) for the conversion of X-rays into electricalsignals, a detector exposure unit (5) for the emission ofelectromagnetic radiation in dependence on first and second exposureparameters, the value of the first exposure parameters being defined bythe acquisition mode whereas the second exposure parameters are notdefined by the acquisition mode, and a control unit for changing andcontrolling at least one second exposure parameter of the detectorexposure unit (5) upon a change of the acquisition mode.
 2. An X-rayapparatus as claimed in claim 1, characterized in that the X-rayapparatus additionally includes an X-ray source (1).
 3. An X-rayapparatus as claimed in claim 1 or 2, characterized in that the controlunit (13) is additionally arranged to control at least one furthercomponent (1, 4) of the X-ray apparatus.
 4. An X-ray apparatus asclaimed in claim 1, characterized in that the image acquisition timeand/or the X-ray exposure time and/or the detector resolution areacquisition parameters, which determine the acquisition mode.
 5. AnX-ray detector as claimed in claim 1, characterized in that the controlunit (13) is arranged to change and control the exposure intensityand/or exposure time and/or exposure wavelength composition of thedetector exposure unit (5).
 6. An X-ray apparatus as claimed in claim 1,characterized in that an exposure parameter is changed and controlled inproportion to the change of an acquisition parameter.
 7. An X-rayapparatus as claimed in claim 1, characterized in that the change andcontrol of an exposure parameter satisfies a function, which convergestowards a constant final value as from the change of the acquisitionmode.
 8. An X-ray apparatus as claimed in claim 1, characterized in thatthe control unit (13) is arranged to read out values for changing andcontrolling the exposure parameters from a storage medium (15).
 9. AnX-ray apparatus as claimed in claim 1, characterized in that thedetector exposure unit (5) is active only within the electronic resetphase of the X-ray detector (4).
 10. A method of converting X-rays intoelectrical signals by means of an X-ray apparatus for forming X-rayimages, which method includes the steps of: irradiating an X-raydetector (4) by means of X-rays, additionally irradiating the X-raydetector, in dependence on first and second exposure parameters, bymeans of a detector exposure unit (5), which emits electromagneticradiation, the value of the first exposure parameters are not defined bythe acquisition mode, changing and controlling at least one of thesecond exposure parameters of the detector exposure unit (5) after achange of the acquisition mode, reading out the electrical signalsproduced by the X-ray detector (4).